There is a lot of on-system documentation available in a typical GNU/Linux (or similar) system. These range from the built-in help in tools, to manual pages, texinfo pages, examples and other packaged documentation in various forms. Knowing how to navigate these troves of information will help you immensely when you are looking for help on a particular part of your system.

Built in help

Most tools will have built in help which will, at least, give you an idea of the commandline syntax for using them. The standard way for tools to offer this to you these days is with a --help or -h commandline argument.

For example:

$ who --help
Usage: who [OPTION]... [ FILE | ARG1 ARG2 ]
Print information about users who are currently logged in.

-a, --all         same as -b -d --login -p -r -t -T -u
-b, --boot        time of last system boot
...

Commands which have been grandfathered in from a time before GNU long options were common may require the -h option form:

$ reset --help
reset: invalid option -- '-'
Usage: tset [options] [terminal]
...

Often this is good enough to get you the help, but sometimes it'll really be confusing. Some commands don't support -h or --help (or they end up mapping them to real arguments) and so you need to do something outlandish to get them to give up their hoarded secrets:

$ ssh-keygen -\?
ssh-keygen: illegal option -- ?
usage: ssh-keygen [options]
Options:
  -A          Generate non-existent host keys for all key types.
  -a trials   Number of trials for screening DH-GEX moduli.
...

Every now and again, a command has sub-commands (for example git, svn or morph) and for those there may be a help subcommand to help you along. For example:

$ svn help ls
list (ls): List directory entries in the repository.
usage: list [TARGET[@REV]...]

  List each TARGET file and the contents of each TARGET directory as
  they exist in the repository.  If TARGET is a working copy path, the
...

Give it a go, explore what commands can tell you about their usage and see what you can learn in only a few minutes.

Manual Pages

Most commandline applications, and indeed many non-commandline apps have manual pages. Manual pages are an old concept and as such they are often well provided for. Unfortunately they are also often written in a special language known as Mannish which can take a while to get used to.

The man command is your primary gateway to the manual pages on your computer. There is also apropos or man -k which can be used to search for appropriate pages to display. Manual pages are usually rendered for your terminal width at the point you invoke the man tool and will be displayed in the pager for you to scroll up and down at will.

$ man ls
... the manual page for ls is displayed ...

or

$ apropos revision
darcs (1)            - an advanced revision control system
git-blame (1)        - Show what revision and author last modified each line of a file
gitrevisions (7)     - specifying revisions and ranges for git
uupdate (1)          - upgrade a source code package from an upstream revision

Whenever manual pages (or indices) refer to other manual pages, they will tend to state not only the name but the section where that page can be found. That is the number in parentheses following the manual page names in the listing above. Traditionally the manual sections have specific meanings...

• 1: Executable programs or shell commands
• 2: System calls (functions provided by the kernel)
• 3: Library calls (functions within program libraries)
• 4: Special files (usually found in /dev)
• 5: File formats and conventions eg /etc/passwd
• 6: Games
• 7: Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
• 8: System administration commands (usually only for root)
• 9: Kernel routines [Non standard]

You will also find unusual manual sections such as 3pm for Perl modules.

If you find that a given name for a manual page is available in multiple sections then you can select which section simply by running man section page. For example, the passwd program has a manpage, as does the passwd file format:

$ man passwd
... passwd(1) is shown...

$ man 5 passwd
... passwd(5) is shown...

Texinfo

GNU introduced a mechanism of offering documentation which was more powerful, hyperlinked and theoretically easier for the author to produce than manual pages. This is the texinfo or info system. Documentation in this form is often referred to as a Texinfo manual and the info program gives you access to it.

Sometimes manual pages will refer you to a Texinfo manual for further information about the topic you are investigating. For example:

$ man ls
...
       The  full  documentation for ls is maintained as a Texinfo manual.
       If the info and ls programs are properly installed at  your  site,
       the command

              info coreutils 'ls invocation'

       should give you access to the complete manual.

The example given (info coreutils 'ls invocation') shows how you can use the info tool to jump to a particular node (ls invocation) in the manual for coreutils.

Navigating in the info tool is pretty simple. You can use the arrow keys to move around the document. Anything with a double colon (::) after it is a link and you can press ENTER when your cursor is over such a link to traverse it. There are also a number of single keypress navigation options:

• SPACE: Page down
• b: Go up a page
• n: Go to the next node
• p: Go to the previous node
• q: Quit the viewer

But perhaps the most important keypress to learn is h which will show you the help for the info viewer.

Packaged help and examples

This is the least well structured of the options available on a typical system. There is no standard way for examples etc to be provided. I shall limit myself to where you can look on Debian and derived systems such as Ubuntu or Linux Mint.

On these systems, each package which is installed places additional documentation in two common places. The first is simply a second package typically called whateverpackage-doc. For example, there is libvdk2-2c2 and its associated libvdk2-dev and libvdk2-doc packages. To get these packages onto your system, simply install them as you would any other.

Whatever package you are interested in, be it a -doc package or not, you can ask dpkg for the list of files in it with dpkg -L packagename and if you pipe that through grep doc then you'll get a good idea of where documentation can be found.

$ dpkg -L subversion | grep doc
/usr/share/doc
/usr/share/doc/subversion
/usr/share/doc/subversion/NEWS.Debian.gz
/usr/share/doc/subversion/copyright
/usr/share/doc/subversion/HACKING
/usr/share/doc/subversion/changelog.gz
...

As you can see, each package will also install a /usr/share/doc/<packagename> directory into which various documentation may be placed. This is a common place to find examples.

$ dpkg -L gnupg | grep example
/usr/share/doc/gnupg/examples
/usr/share/doc/gnupg/examples/convert-from-106

There's clearly many other places where documentation might lurk. Some of it might show in things like the GNOME documentation index displayed if you run gnome-help. Simply examine the contents of packages to see where they might be hiding documentation and you'll rapidly find it.

Conclusion

There is a significant amount of documentation available on your typical GNU/Linux (or similar) system. There's often no need to go to Google to find what you need to know. Just learn how to navigate what you already have.